First off a fresh install is always nicer than an upgrade, might be worth giving that a shot if it's a possibility.
Secondly, you need to find where the problem lies. Try runnning some simple pings to public sites such as eveonline.com, google.com etc and see if your getting any packet loss and the average latency. If these show a problem then either your networking hardware is at fault (doubtful if your PS3 works through the same router), your network driver is faulty/bugged or you have some security program not doing it's job properley.
If your pings come back fine then it's likely the problem exists higher up in the stack, i.e. software/application level. Have you tried booting in safe mode and trying to browse the web and see what your performance is like?
I assume of course you've scanned your entire PC for virus' and Malware? They can also severely hamper performance (along with stealing your details
Failing that, replace your security software with something else such as AvG, Avast, malware bytes, Spybot etc to see if it's that.
Give those things a shot and post back with the result and any extra info you find along the way :)
P.s. you might get a better response posting this in the'Issues and workarounds' forum